Check in codegen test expectations.

test/codegen/expect/language/mixin_regress_13688_test.js

Index: test/codegen/expect/language/mixin_regress_13688_test.js
diff --git a/test/codegen/expect/language/mixin_regress_13688_test.js b/test/codegen/expect/language/mixin_regress_13688_test.js
new file mode 100644
index 0000000000000000000000000000000000000000..f22f830f0b39fa507798f07279bcbe8e075d522c
--- /dev/null
+++ b/test/codegen/expect/language/mixin_regress_13688_test.js
@@ -0,0 +1,34 @@
+dart_library.library('language/mixin_regress_13688_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__mixin_regress_13688_test(exports, dart_sdk, expect) {
+  'use strict';
+  const core = dart_sdk.core;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const expect$ = expect.expect;
+  const mixin_regress_13688_test = Object.create(null);
+  let ComparableMixin = () => (ComparableMixin = dart.constFn(mixin_regress_13688_test.ComparableMixin$()))();
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  mixin_regress_13688_test.ComparableMixin$ = dart.generic(E => {
+    class ComparableMixin extends core.Object {
+      e() {
+        return dart.wrapType(E);
+      }
+    }
+    dart.addTypeTests(ComparableMixin);
+    dart.setSignature(ComparableMixin, {
+      methods: () => ({e: dart.definiteFunctionType(dart.dynamic, [])})
+    });
+    return ComparableMixin;
+  });
+  mixin_regress_13688_test.ComparableMixin = ComparableMixin();
+  mixin_regress_13688_test.KUID = class KUID extends dart.mixin(core.Object, mixin_regress_13688_test.ComparableMixin$(mixin_regress_13688_test.KUID)) {};
+  mixin_regress_13688_test.main = function() {
+    let kuid = new mixin_regress_13688_test.KUID();
+    expect$.Expect.equals(dart.toString(kuid.runtimeType), dart.toString(kuid.e()));
+  };
+  dart.fn(mixin_regress_13688_test.main, VoidTodynamic());
+  // Exports:
+  exports.mixin_regress_13688_test = mixin_regress_13688_test;
+});